The differential effect of illicit drug use on cognitive function in first-episode psychosis and healthy controls. Objective: Illicit drug use can result in impairment in cognitive function in healthy individuals. Individuals with a psychotic disorder also show a deficit in cognitive function. Drug use may simply contribute to the characteristic cognitive deficit found in psychosis or alternatively result in a 'double deficit'. This study aims to investigate the association between drug use and cognitive function at the first-episode of psychosis and in community-matched controls. Method:: One hundred and seventy-seven patients at the first episode of psychosis completed a battery of neuropsychological tests. Those that had used drugs in the previous year (n = 80) were compared with those who had not used drugs in the previous year (n = 97). A subsample of the first-episode psychosis patients were compared with community-matched controls (n = 110) according to drug-use status. Results:: Patients with a first episode of psychosis who had used drugs performed equally to those who had not used drugs on neuropsychological tests. In contrast, healthy controls who had used drugs in the previous year performed worse on tests of executive function and working memory compared with those controls that had not used drugs. Conclusion: There are differential associations of illicit drug misuse with cognitive function for first-episode psychosis patients and healthy controls.